The present invention relates to a method for controlling a matrix converter with nine bidirectional power switches arranged in a 3xc3x973 matrix, employing a space vector modulation process to calculate switching states in a modulation period and time intervals associated with the modulation period.
A matrix converter is a self-commutated direct converter and enables the conversion of a constant three-phase system into a system with variable voltage and frequency. Through the arrangement of the bidirectional power switches in a 3xc3x973 switch matrix, each of the three output phases of the matrix converter can be electrically connected to any one input phase. One phase of the matrix converter includes an arrangement of three bidirectional power switches, whereby each power switch is connected to an input phase and, on one hand, and to an output phase, on the other hand. An arrangement of this type is also referred to as a 3xc3x971 switch matrix. The matrix converter does not require an intermediate circuit. Due to its topology, the self-commutated direct converter has a recovery capability and achieves sinusoidal mains currents through a suitably designed control.
Each of the bidirectional power switches of the matrix converter has two anti-serially connected semiconductor switches. Insulated Gate Bipolar Transistors (IGBT) are preferably used as semiconductor switches, whereby each of the semiconductor switches includes an antiparallel diode. Bidirectional power switches designed in this way are preferably used in converters for low and medium power. Through the control of these semiconductor switches of the bidirectional power switches, a continuous current path is established in a direction determined by the arrangement of the semiconductor switches. In the event both semiconductor switches of a bidirectional power switch are controlled, the latter is bidirectionally activated and a current can flow in both directions. This creates a safe electrical connection between an input phase and an output phase of the matrix converter. When only one semiconductor switch of a bidirectional power switch is controlled, the latter is unidirectionally activated, creating an electrical connection between an input phase and an output phase of the matrix converter only for a preferred current direction.
Any desired time-averaged output voltage can be obtainedxe2x80x94within certain limitsxe2x80x94by a controlled time sequence of combinations of switch positions within a modulation period. A matrix converter includes a controller capable of computing a suitable switch combination based on information about the input voltage space vector and a desired value for the output voltage space vector.
Conventional control methods operate either according to a phase-oriented method or a vector-oriented method.
The phase-oriented control method is described in the publication xe2x80x9cAnalysis and Design of Optimum-Amplitude Nine-Switch Direct AC-AC Convertersxe2x80x9d, by Alberto Alesina and Marco G. B. Venturini, IEEE Transactions on Power Electronics, Vol. 4, No. 1, January 1989, pp. 101-112. The space vector control method is described in xe2x80x9cSpace Vector Modulated Three-Phase to Three-Phase Matrix Converter with Input Power Factor Correctionxe2x80x99, by Lxc3xa1szxc3x3 Huber and Du{umlaut over (s)}an Borejevixc4x87, IEEE Transactions on Industrial Applications, Vol. 31, No. 6, November/December 1995, pp. 1234-1245. The space vector modulated control method has significant disadvantages due to high switching losses. Segments are present in the output voltage and the input current which have a pulse frequency equal to the modulation frequency, with other segments having twice the pulse frequency.
The publication xe2x80x9cSpace Vector Modulated Matrix Converter with Minimized Number of Switchings and a Feedforward Compensation of Input Voltage Imbalancexe2x80x9d, by P. Nielsen, F. Blaabjerg, and J. K. Pedersen, Proceedings of the 1996 International Conference on Power Electronics, Drives and Energy Systems for Industrial Growth, pp. 833-839, discloses a method for reducing the number of commutations. With his method, four active switching states and one switching state which generates at the output of the matrix converter a voltage space vector with Zero amplitude, are calculated using a space vector modulation method. The switching states are referred to in this publication as active vectors and as Null vector. During the space vector modulation of a matrix converter, the input current vector and the output voltage vector can be located in the same sector or in neighboring sectors. Other combinations are possible in addition to the aforedescribed combinations. The pulse frequency, i.e. the voltage space vector sequence, is usually configured symmetrically, with the null vector being located in the center of the four active vectors. If the input current vector and the output voltage vector of the matrix converter are located in the same sector, then the pulse sequence results in eight commutations. Conversely, if the input current vector and the output voltage vector of the matrix converter are located in adjoining sectors, then the pulse sequence results in ten commutations, without optimization. By using the optimization proposed in the reference, a pulse sequence is generated which has also only eight commutations. The optimized pulse sequence is obtained by combining the calculated four active vectors and a null vector. The optimized pulse sequence differs from the non-optimized pulse sequence in that the time sequence of the pulses of the active vectors is reversed and a suitable Null vector is selected. The null vector is selected from the three possible null vectors in such a way that only one commutation takes place. With this optimized space vector modulation method, only eight commutations occur during each modulation period. Reducing the number of commutations per modulation period also reduces the switching losses of the matrix converter.
This control method has an additional disadvantage regarding switching losses, since the optimization of the space vector modulation method only considers the number of commutations, but not the voltages at which the commutations take place.
It would therefore be desirable and advantageous to provide an improved method for controlling a matrix converter, which obviates prior art shortcomings and further reduces switching losses in the matrix converter.
According to one aspect of the present invention, a method for controlling a matrix converter, with nine bidirectional power switches arranged in a 3xc3x973 switch matrix, includes the steps of calculating with a space vector modulation method switching states of a modulation interval with corresponding time intervals, decomposing the calculated switching states into corresponding output-phase-related switching states of the matrix converter; associating time intervals with the output-phase-related switching states, wherein the time intervals and the output-phase-related switching states are placed into one-to-one correspondence, andxe2x80x94depending on the measured input voltagesxe2x80x94combining the output-phase-related switching states with the associated time intervals to form a pulse sequence of a modulation period in such a way that a sequential commutation is always performed to a nearest input voltage.
The order in which the output-phase-related switching states are sequentially arranged depends on the input voltages. The switching losses of the matrix converter are reduced significantly by optimizing the pulse sequence based on the input voltages within a modulation period.
The output-phase-oriented re-sorting of calculated switching states of the matrix converter produces a switching state which does not occur among the initially computed switching states of a modulation period. With the switching state, each output phase of the matrix converter is connected with an input phase that is different from the input phases of the other output phases. Because of its circular trajectory, an associated output voltage space vector is referred to as a xe2x80x9crotating space vector.xe2x80x9d This switching states is excluded in a space vector modulation due to its small maximum output voltage. However, when using the control method of the invention, the same switching states is produced within a modulation period, without exhibiting this disadvantage.